Extending the Direct Strength Method for Cold-Formed Steel Design to Through-Fastened Simple Span Girts and Purlins with Laterally Unbraced Compression Flanges

Abstract

A direct-strength method (DSM) prediction approach is introduced and validated for metal building wall and roof systems that are constructed with steel panels through-fastened with screws to girts or purlins. The focus is capacity prediction for simple spans under wind uplift or suction; however, the DSM framework is generally formulated to accommodate gravity loads, continuous spans, standing-seam roofs, and insulated roof and wall systems in the future. System flexural capacity is calculated with the usual DSM approach; global buckling, local-global buckling interaction, and distortional buckling strengths are determined with a finite-strip Eigen-buckling analysis, including a rotational spring that simulates restraint provided by the through-fastened steel panel. The DSM flexural capacity is then reduced with a code-friendly equation consistent with existing standard provisions to account for the additional stress at the intersection of the web and free flange that occurs as the girt or purlin rotates with respect to a suction (uplift) load. A database of 62 simple-span tests was assembled to evaluate the strength prediction accuracy of the proposed DSM approach alongside existing standard provisions. The proposed DSM approach is confirmed to be viable and accurate for simple spans. Modifications to a standard approach are proposed that could improve its accuracy. The R-factor prediction method is accurate for c-section simple spans, unconservative for z-section simple spans, and overall lacks the generality of the other two approaches reported in this paper.